1. Field of the Invention
The present invention relates to a gallium phosphide light-emitting diode and particularly, relates to a gallium phosphide light-emitting diode which emits green light.
2. Description of the Prior Art
Conventionally, in a light-emitting diode using gallium phosphide GaP, a method of decreasing the concentration of an n-type layer of pn junction has been known as an effective method for emitting green light with high efficiency. In page 537 of Toshiba Review Vol. 37 No. 6, it is reported that electroluminescence efficiency of approximately 0.3% can be stably obtained if an n-type layer is divided into two layers so that impurity concentration has values changed in steps. This will be described in the following with reference to the drawings. FIG. 1 is a view showing typically a conventional gallium phosphide light-emitting diode. FIG. 2 is a schematic view showing distribution of impurity concentration for each portion of the light-emitting diode in FIG. 1. A conventional gallium phosphide light-emitting diode 1 comprises an n-type layer 11 formed on an n-type substrate 10, an n-type layer 12 formed on the n-type layer 11 and a p-type layer 13 formed on the n-type layer 12. The impurity concentration for the n-type layers 11 and 12 is changed in steps, as shown in FIG. 2. However, such distribution of impurity concentration often causes switching operation (that is, operation as a thyristor or as a transistor). An experiment was made to investigate the cause, and in consequence, it has become clear that in case of simply decreasing the concentration of the n-type layer near the pn junction, there exist a large amount of electric current and other factors which do not serve to emit light in the n-type layer and that certain impurities act as a minority carrier to cause switching operation. More particularly, in the n-type layer, regions where the acceptor concentration is higher than the donor concentration are produced and p-type inversion is made only in such regions. Accordingly, undesirable npnp junction or npn junction is produced, which causes switching operation. Therefore, it has proved to be necessary to grow n-type layers adjusting the perfection of crystal (especially lattice matching or dislocation in the regions of different impurity concentrations), as well as to decrease the impurity concentration of the n-type layer near the pn junction.